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Sammanfattning
Så länge det har funnits vete har det förmodligen funnits patogener som orsakar sjukdomar på veteplantorna. Flera av dem är svampar även om virus, bakterier, och nematoder också tar sin näring från plantorna. Forskare har teorier om att vissa svampsjukdomar uppstod i samband med att vetet för tolv tusen år sedan började odlas i ”bördiga halvmånen” i Mellanöstern (”the Fertile Crescent”). När utbredningen av veteodlingen drogs norrut i Europa följde patogenerna med via utsädet elleer spreds via vindburna sporer. Under 1800-talet identifierades flera patogena svampar som angriper vete, mycket tack vare utvecklingen av mikroskopet. Förädlingen av vetesorter under 1900-talet ledde till plantor med längre ax, kortare strå och breda blad för att öka skördarna. Frodiga bestånd medförde att skördeförlusterna på grund av bladfläcksvampar ökade eftersom de gynnas av ett fuktigt mikroklimat. Ett kort avstånd mellan bladnivåerna förkortar dessutom spridningsavståndet för svamparna.
Svamppopulationen på veteblad med nekrotiska fläckar
Veteblad med nekrotiska fläckar kan härbärgera flera arter av svampar.
Tretton arter och ett artkomplex identifierades från höstveteblad insamlade i elva fält fördelade i Östergötland, Västergötland och Uppland under tre år (Tabell 1, Figur 5). Tre arter var patogena, tre var saprofyter och resten var olika arter av jäst. Jästsvamparna förekom i alla fält vilket tyder på att de samexisterar med de patogena svamparna. Troligen har jästsvamparna inte så stor betydelse för grödan.
De patogena svamparna var Mycosphaerella graminicola (anamorph Septoria tritici) som orsakar svartpricksjuka, Pyrenophora tritici-repentis (anamorph Drechslera tritici-repentis) som orsakar vetets bladfläcksjuka samt Phaeosphaeria nodorum (anamorph Stagonospora nodorum) som orsakar brunfläcksjuka i vete.
P. nodorum dominerande i flera av fälten under 2003 och 2004 medan M.
graminicola var vanligast i både Östergötland och Västergötland under 2005. I Uppland dominerade P. tritici-repentis under 2005 medan P. nodorum fanns i mycket liten mängd (I). Symptomen av brunfläcksjuka (Figur 1) och vetes bladfläcksjuka (Figur 4) är förväxlingsbara under senare sjukdomsstadier. Det bästa sättet att skilja svamparna åt är att studera de asexuella sporerna, konidierna (Figur 3).
Biologin hos Phaeosphaeria nodorum
I livscykeln för P. nodorum finns en asexuell och en sexuell fas (Figur 2). Den asexuella fasen innefattar bildandet av asexuella sporkapslar, pyknider, på blad, ax och och halmrester. Pykniderna innehåller konidier som tränger ut vid fuktig väderlek. De sprids via regnstänk mellan bladen i beståndet.
Bildandet av pyknider och konidiespridning kan ske flera gånger under en växtsäsong. Om svampen når axet etablerar den sig på kärnorna. Vid skörd förs svampen vidare med utsädet till nästa odlingssäsong. P. nodorum kan också övervintra på växtrester i fältet. Cirkeln är sluten om konidier från pyknider som bildas på halmresterna kan infektera nya plantor året därpå.
Smittat utsäde medför att veteplantans tillväxt hämmas eller att plantan dör.
Förekomsten av det sexuella stadiet hos P. nodorum i Sverige bekräftades vid en analys av svampens genetiska struktur (II). I den sexuella fasen av livscykeln möts två individer med parningstyperna 1 respektive mat1-2. De båda parningstyperna förekommer lika frekvent i fälten och de är slumpmässigt fördelade, vilket ökar möjligheterna för sexuell reproduktion.
Resultatet av den sexuella reproduktionen är en fruktkropp, pseudotecium, som innehåller ascosporer. Ascosporerna är vindburna vilket medför att de kan spridas till nya veteplantor såväl inom som mellan fält.
Olika individer av en svamp har oftast olika antal upprepningar och olika kombinationer av mikrosatelliter, vilka tillsammans med parningstypen utgör isolatets genotyp. Totalt identifierades 93 olika genotyper bland 227 isolat, varav 69 förekom endast en gång. Inom varje fält fanns det mellan 23 och 38 genotyper. Många unika genotyper tillsammans med slumpvis förekomst av parningstyperna i fält är en mycket stark indikation på att P. nodorum förökas på sexuell väg även i Sverige.
Olika isolat av P. nodorum har varierande nivå av aggressivitet mot vetesorter (III). Sex höstvetesorter som provades i ett aggressivitetstest hade ungefär samma mottaglighet för angrepp av brunfläcksjuka. Latensperioden för svampen förkortades då plantorna inokulerades med hög konidiekoncentration.
Bekämpning av växtpatogena svampar
Angreppen av P. nodorum kan förhindras eller bekämpas på flera sätt. En bra växtföljd där stråsäd alterneras med t.ex. oljeväxter och ärtväxter samt god jordbearbetning med framförallt plöjning, minskar halmmängden på markytan och därmed möjligheterna för inokulum att överleva och orsaka nya infektioner. Fungicider förhindrar myceltillväxt och sporgroning, genom att störa en viktig enzymaktivitet. Under 2000-talet har lantbrukare i Skåne och Danmark, men även i andra delar av Europa, märkt en försämrad effektivitet av fungicidbehandlingar mot M. graminicola och P. tritici-repentis.
Orsaken är att någon av de aminosyror som bygger upp enzymet som substansen ska fästa vid har bytts ut, varvid fungiciden inte längre kan störa enzymet i fråga. Fyra substansers effektivitet mot P. nodorum undersöktes genom odling på agarplattor (IV). Cyprodinil som tillhör gruppen anilinopyrimidiner, samt propiconazole och prothioconazole, tillhörande gruppen triazoler var effektiva mot P. nodorum. Azoxystrobin, som är en strobilurin, hade delvis förlorat förmågan att hämma tillväxten av P. nodorum.
Majoriteten av isolaten hade ett EC50-värde (den effektiva koncentrationen som minskar tillväxten till 50 % av referensisolatens tillväxt) som var högre än referensisolatens genomsnittliga EC50-värde. Vid sekvensering av den gen som kodar för cytokrom b, det protein som azoxystrobin ska fästa vid, hade majoriteten av isolaten aminosyran alanin istället för glycin vid position 143.
Substitutionen som betecknas g143a, fanns hos 79 % av de 227 P. nodorum-isolat som sekvenserades. I ett fält var förekomsten av substitutionen endast 50 % bland de insamlade isolaten. Detta är den första rapporten någonsin om att P. nodorum har blivit okänslig mot strobiluriner.
Slutsats
Dessa resultat har betydelse för lantbruket i flera avseenden. I ett epidemiologiskt perspektiv är kunskapen om att P. nodorum kan spridas mellan fält via ascosporer och ge upphov till angrepp trots god växtföljd och jordbearbetning viktig för valet av odlingsstategier. En stor del av den svenska P. nodorum-populationen är okänslig mot strobiluriner and den andelen kan öka ytterligare genom selektion vid fungicidbehandling. Detta innebär att strobiluriner ej bör rekommenderas för behandling mot bladfläcksvampar i vete. Aktsamhet bör även iakttas vid användning av andra fungicider eftersom andra patogena svampar har förlorat sin känslighet mot exempelvis triazoler.
Acknowledgements
I know that I have a long list of people of importance for me and my journey on the sea of science, so I will just start expressing gratitude to my supervisors: Jonathan Yuen, the captain on the ship, has always been dedicated and calm, always thinking that everything will solve in one way or another. Annika Djurle, the officer of the ship, gave me the first plant pathology impression when she in 2001 gave a lecture on some aspects of plant pathology during historical events: battles and famines that may have changed the history due to plant pathogens! Åke Olson joined this great crew after the emigration of us, agricultural plant pathologists, to the Dept.
of Forest Mycology and Pathology. Åke have served as a helmsman on this ship with his knowledge of the wonders of genetics. His encouragement and inspiration has helped me through this journey. I have also much enjoyed our short meetings on one of the 62 steps in the winding stairway of the Mykopat building. Berit Samils started up with us when we set sail, but transformed unfortunately to a land crab after two years of sailing. I had two other important sailors onboard this ship: Nils Högberg and Björn Lindahl, who showed me the paradisiacal islands of the population genetics and fungal community.
My mentor, Docent Anders Göransson. Thank you for the inspiring and encouraging talks during our mushroom-collecting walks in the forest.
The laboratory work would have been much harder without the help from Lena F, Anna-Karin W, Katarina, Maria J, and Eva F along with the deckshand David and some youngsters who helped me through the fungicide sensitivity test. My dear computer was recovered several times by Per N, Les or Patrik.
I have much to thank the people at the Swedish Board of Agriculture, subdivision Plant Protection Centres, the farmers of the sampled fields and
agricultural practice. I would also like to thank Ingrid Happstadius (Svalöf Weibull AB) for kindly contributing with seeds and with one isolate of P.
nodorum.
There are a few people who have inspired me to endeavour the tremendous world of plant pathology: Prof. Jan Zadoks, who I first met in Tunis in 2003 and has since then inspired me to the interesting topics of epidemiology and the history of plant pathology, Prof. Bruce McDonald who guided me when I first dove into the ocean of fungal genetics, Dr Alison Lees, Prof. Anne-Marte Tonsmo and Dr Mette Lübeck who introduced me to the marvellous world of diagnostics. Now I think that I have found my avocation!
Docent Petra Fransson, thank you for all the nice talks in the coffee room and elsewhere on topics ranging from horses to deep scientific questions.
My dear IMOP colleagues, Kalle, Alexandra, Greg, Hasna, Jon, Jesper, Mattias, Maria K, who also was a fine delegate in the post graduate student council and Dharam, with whom I shared room for more than two years.
I have spent some nice years as a member of the Postgraduate student council. Thank you Johanna W, Eva-Lotta, Per H, Johan, Karin P, Doktorandombudsman Hans Arrendahl and many more PhD students.
Together we have achieved many nice things that we should be proud of.
Ylva P for being such a nice roommate and that you and Inga took over the important role of being a member of the post graduate student council.
Good luck to you both and keep improving the situation for the post graduate students.
Many thanks to all my companying Mykopats (some mentioned, none forgotten) and the seamen at the former Dept of Ecology and Crop Production that have come and gone through the years: Johanna B (S), Maria V, Maria B, Hanna F, Johan, Liv, and the list can be longer. CG, thank you for all the great ”spexs” of which we were a part of the creation. I can soon compete with you on thanking the highest number of people in the Acknowledgement. Mine is at least longer that yours…
Some important people that have served as my rescue boat are my dear friends Maria S, Anna M, Linda W, Juliane, Hubbe, Tojje, Magnus J, Nisse M, Sanna, Ewa W, Annelie, Niklas I, Kristina and my friends in the brass band, in the stable and my sparring partner Margareth, for helping me to clear my thoughts and lighten up my weekdays. Nicklas S, you are so cool and you always stand up for yourself and other people. Keep improving your karma and you will soon be there.
Ett särskilt stort tack vill jag rikta till klanen Johansson på Öland med omnejd, speciellt till Er på Kulltorp, för att Ni ha fått mig att bejaka mitt agrara ursprung och satsa på agronomyrket. Länge leve det svenska jordbruket! Jag vill också tacka för pratstunderna med moster Stina för att hon oftast svarar när ingen annan gör det. Till mina släktingar på pappas sida vill jag säga att det har det varit en fröjd att ta upp umgänget igen i och med min flytt till Uppsala.
Och så vill jag tacka mina kära svärföräldrar Barbro och Hans och svåger Johan för att Ni på Ert sätt har varit delaktiga i den här processen. Hur många kan svära(?) över att svärdotra har dragit in mögel i källaren under tre somrar?
Mamma och Erik, jag dedikerar avhandlingen till Er, det säger allt. Kram från dotra och storasyrran.
I will send a great amount of gratefulness to my beloved husband Niclas, who has stood by me for ten years now this Christmas. I can’t express my gratitude to you. It is so relieving to come home and the fragrance of food greets me in the hallway. I enjoyed our nice sampling trips through the lovely sceneries of the dazzling Sweden during summer. You were tremendous in handling the spore trap so that each sampling occasion became a few hours shorter, much appreciated during a hot summer’s day.
Yes, that´s it for now. I apologise for those that I might have forgotten but you will all be in my heart forever. May peace be with you and have a nice future.
Yours truly, Eva
The projects were funded by Signe och Olof Wallenius Stiftelse, C. F.
Lundströms Stiftelse, the Swedish University of Agricultural Sciences (slu) and the postgraduate school imop (Interactions between Micro-Organisms and Plants) at slu.